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United States Patent |
6,205,269
|
Morton
|
March 20, 2001
|
Optical add/drop multiplexer
Abstract
An add/drop multiplexer includes first and second fiber grating connected
to a four-port optical circulator having first and second input/output
ports, an add port, and a drop port, wherein the first fiber grating is
connected to the first input/output port and the second fiber grating is
connected to the second input/output port. Add signals provided to the add
port of the circulator are reflected by the first fiber grating back to
the optical circulator and propagate, along with one or more input optical
signals, to the second fiber grating. One or more of the input optical
signals are reflected by the second fiber grating back to the optical
circulator and propagate to the drop port of the circulator.
Inventors:
|
Morton; Paul A. (West Friendship, MD)
|
Assignee:
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Ciena Corporation (Linthicum, MD)
|
Appl. No.:
|
286848 |
Filed:
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April 6, 1999 |
Current U.S. Class: |
385/24; 385/37; 398/9 |
Intern'l Class: |
G02B 006/28 |
Field of Search: |
385/24,37
359/124,127,130
|
References Cited
U.S. Patent Documents
5712717 | Jan., 1998 | Hamel | 359/130.
|
5717798 | Feb., 1998 | Strasser | 385/37.
|
5748349 | May., 1998 | Mizrahi | 359/130.
|
5748350 | May., 1998 | Pan | 359/130.
|
5822095 | Oct., 1998 | Taga | 385/24.
|
5982518 | Nov., 1999 | Mizrahi | 385/24.
|
6041152 | Mar., 2000 | Clark | 385/24.
|
6067389 | May., 2000 | Fatehi | 385/17.
|
Foreign Patent Documents |
0794629A2 | Sep., 1997 | EP.
| |
0857988A1 | Aug., 1998 | EP.
| |
Primary Examiner: Spyrou; Cassandra
Assistant Examiner: Boutsikaris; Leo
Attorney, Agent or Firm: Daisak; Daniel N., Soltz; David L.
Claims
What is claimed is:
1. An optical add/drop multiplexer comprising:
a first fiber Bragg grating positioned along an optical waveguide, said
first grating configured to reflect a first optical signal operating on a
first wavelength and transmit a second optical signal operating on a
second wavelength to a first grating output;
an optical circulator having a first and second input/output ports, an add
port and a drop port, said first input/output port optically communicating
with said first grating output and configured to receive said second
optical signal, said add port configured to receive a third optical signal
operating on said first optical wavelength, said second input/output port
outputting said second and third optical signals; and
a second fiber Bragg grating optically communicating with said second
input/output port and receiving said second and third optical signals,
said second fiber grating reflecting said second optical signal back to
said second input/output port and transmitting said third optical signal,
said second optical signal exiting said drop port of said optical
circulator.
2. The optical add/drop multiplexer in accordance with claim 1 wherein said
first fiber grating has a stop-band wherein said first wavelength lies
within said stop-band.
3. The optical add/drop multiplexer in accordance with claim 1 wherein said
second fiber grating has a stop-band wherein said second wavelength lies
within said stop-band.
4. The optical add/drop multiplexer in accordance with claim 1 wherein said
second fiber grating has a stop-band wherein said third wavelength falls
out7side said stop-band.
5. An optical communication system comprising:
a first optical waveguide carrying a wavelength division multiplexed
optical signal having a plurality of optical channels each at a respective
wavelength;
a first fiber grating positioned along said first optical waveguide, said
grating configured to receive said first wavelength division multiplexed
optical signal and passing said first wavelength division multiplexed
optical signal to a first fiber grating output;
an optical circulator having a first input/output port, an add port, a drop
port and a second input/output port, said first input/output port
optically communicating with said first fiber grating output and receiving
said first wavelength division multiplexed optical signal, said add port
receiving an optical add signal operating on a first optical wavelength,
said drop port outputting one of said plurality of optical signals in said
first wavelength division multiplexed optical signal, said second
input/output port outputting said plurality of optical signals in said
first wavelength division multiplexed optical signal and further
outputting said optical add signal;
a second fiber grating optically communicating with said second
input/output port of said optical circulator, receiving the first
wavelength division multiplexed optical signal and said optical add
signal, said second fiber grating reflecting back to said second
input/output port said one of said plurality of optical signals in said
first wavelength division multiplexed optical signal output by said drop
port, and outputting said plurality of optical signals in said first
wavelength division multiplexed optical signal other than said one of said
plurality of optical signals output by said drop port, and further
outputting said optical add signal; and
a second optical waveguide optically communicating with said second fiber
grating and carrying a second wavelength division multiplexed optical
signal comprising said first wavelength division multiplexed optical
signal other than said one of said plurality of optical signals output by
said drop port, and said optical add signal.
6. An optical add/drop multiplexer, comprising:
a first fiber grating for receiving a first optical signal operating on a
first optical wavelength, said first grating configured to transmit said
first optical signal to a first fiber grating output;
an optical circulator having a first input/output, an add port, a drop port
and a second input/output port, said first input/output port optically
communicating with said first fiber grating output and receiving said
first optical signal, said add port configured to receive a second optical
signal operating on a second optical wavelength, said drop port configured
to output said first optical signal, said second input/output port
configured to output said first and second optical signals; and
a second fiber grating optically communicating with said second
input/output port and receiving said first and second optical signals,
said second fiber grating configured to reflect said first optical signal
back toward said second input/output port and transmit said second optical
signal.
7. The optical add/drop multiplexer in accordance with claim 6 wherein said
second fiber grating has a stop-band wherein said first wavelength lies
within said stop-band.
8. The optical add/drop multiplexer in accordance with claim 6 wherein said
second fiber grating has a stop-band wherein said second wavelength falls
outside said stop-band.
9. The optical add/drop multiplexer in accordance with claim 6 wherein said
first fiber grating is configured to reflect a third optical signal
operating on said second optical wavelength.
10. The optical add/drop multiplexer in accordance with claim 6 wherein
said first fiber grating is configured to reflect said second optical
signal operating at said second wavelength back toward said first
input/output port of said circulator.
11. The optical add/drop multiplexer of claim 6, wherein said first fiber
grating has a stop-band wherein said second wavelength lies within said
stop-band.
Description
FIELD OF THE INVENTION
This invention pertains to the field of wavelength division multiplexed
optical communication systems and, more particularly, to an add-drop
multiplexer for transferring selected optical channels between
transmission paths within a wavelength division multiplexed optical
communication system.
BACKGROUND OF THE INVENTION
Wavelength Division Multiplexing (WDM) techniques have been utilized to
significantly enhance the signal capacity of optical communication
systems. WDM systems simultaneously transmit multiple information signals
on a single waveguide medium at different wavelengths or channels.
Examples of such communication systems include, telecommunications
systems, cable television systems, local area networks (LANs) and wide
area networks (WANs). In a WDM system, optical signals are generated and
multiplexed onto a plurality of optical channels, transmitted over a
single optical waveguide, and demultiplexed at one or more destination
terminals. Dense WDM (DWDM) systems are characterized by relatively close
spacings between the respective channels.
WDM or DWDM communication systems may carry signals over many miles, with
the system having a number of different origination and destination
terminals or nodes. In many of these systems, channels are added/dropped
from the WDM signal corresponding to one or more different
origination/destination nodes. This form of optical signal routing is
generally referred to as "add/drop multiplexing."A number of different
devices and configurations have been employed as add/drop multiplexers.
One approach is explored in Giles and Mizrahi, "Low-Loss ADD/DROP
Multiplexers for WDM Lightwave Networks," IOOC Technical Digest, (The
Chinese University Press, Hong Kong) c. 1996, pp. 65-67, the disclosure of
which is incorporated herein by reference. In this paper, an add-drop
multiplexer is proposed which uses two three-port optical circulators with
a fiber grating positioned therebetween. Using this configuration, an
optical signal to be dropped from multiplexed optical signals is reflected
by the fiber grating and exits through a drop port of the first optical
circulator. All other input signals exit via a through port of the first
optical circulator. Similarly, an optical signal to be added which has a
wavelength nominally identical to the optical signal being dropped from
the optical transmission path is input to an add port of the second
circulator. The signal to be added to the optical transmission path is
reflected towards a through port of the second circulator by the same
fiber grating disposed between the first and second circulators used for
signal dropping.
A disadvantage associated with this type of add/drop multiplexer is the
loss associated with the grating and, more significantly, the loss
contributed by each of the two circulators. These loss values have a
negative effect on the overall system loss budget, i.e. the total amount
of optical loss that a given optical link can tolerate while maintaining
signal integrity. Moreover, this loss may accumulate over a plurality of
nodes each including one or more add/drop multiplexers.
Accordingly, it would be advantageous to provide an optical add/drop
multiplexer with reduced optical loss adaptable for use with an optical
communication system employing wavelength division multiplexing. Other and
further objectives will be apparent from the following detailed
description and the appended claims.
SUMMARY OF THE INVENTION
The present invention provides an optical add/drop multiplexer for use in
optical communication systems which includes an optical waveguide capable
of carrying one or more optical input signals on one or more optical input
channels. A first optical fiber grating having an associated stop-band and
positioned along the waveguide reflects a particular channel corresponding
to the optical channel to be added to the input signals. An optical
circulator with four ports optically communicates with the waveguide. The
optical input signals not dropped by the first fiber grating, pass through
the first grating to a first one of the input/output ports of the optical
circulator. The circulator passes the optical input signals to a second
input/output port. The second input/output port is connected to a second
optical fiber grating. The second optical fiber grating reflects those
optical input signals having wavelengths within an associated stop-band,
while other optical signals, outside the stop-band, pass through the
grating. The optical signals reflected by the second grating exit the
circulator through a drop port. In another aspect, an optical circulator
receives an optical add signal at an add port and passes the optical add
signal to the second input/output port, while preventing the optical add
signal from passing to the drop port.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an optical add/drop multiplexer according
to one or more aspects of the present invention.
FIG. 2 is transmission spectrum of an exemplary Bragg grating in accordance
with the present invention.
FIG. 3 is a diagram of an optical communication system which employs an
optical add/drop multiplexer according to one or more aspects of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 schematically illustrates a low-loss optical add/drop multiplexer
200 in accordance with the present invention. The optical add/drop
multiplexer 200 comprises a first fiber Bragg grating 210, an optical
circulator 220, and a second fiber Bragg grating 230. Gratings 210 and 230
comprise a series of photoinduced refractive index perturbations in an
optical fiber which reflect optical signals within a selected wavelength
band, referred to as a "stop-band." The stop band is generally centered at
the Bragg wavelength defined as .lambda..sub.BRAGG =2n.LAMBDA., where n is
the modal index and .LAMBDA. is the grating period. Bragg gratings
suitable for use in the optical device in accordance with the present
invention are described in Morey et al., "Photoinduced Bragg Gratings in
Optical Fibers ," Optics and Photonics News, February 1994, pp. 8-14, the
disclosure of which is incorporated by reference herein.
Circulator 220 has a first input/output port 222, a second input/output
port 224, a drop port 226, and an add port 228. The first grating 210 is
positioned along optical transmission path 205 which optically
communicates with first input/output port 222. One or more input optical
signals are supplied to first port 212 of grating 210 via transmission
path 205. FIG. 2 illustrates an exemplary frequency dependent transmission
characteristic associated with grating 210 wherein optical channels having
wavelengths which fall outside a stop-band pass through the grating and
optical channels having wavelengths which fall within the stop-band are
reflected by grating. For example, FIG. 2 illustrates a transmittance vs.
wavelength spectrum 270 associated with exemplary fiber Bragg grating 210.
As can be seen from curve 270, grating 210 is configured to reflect input
optical signals having wavelengths falling within stop-band region 275.
The optical channels having wavelengths at or very close to .lambda..sub.i
where .lambda..sub.i is the channel to be added to the input optical
signals fall within stop-band 275 and are reflected by the grating.
Optical channels having wavelengths which fall outside stop-band 275 pass
through grating 210 to second port 214. The signals which pass through
grating 210 are supplied to the first input/output port 222 of circulator
220. These signals propagate within circulator 220 and exit via second
input/output port 224.
Second fiber Bragg grating 230 is positioned along transmission path 206
which optically communicates with port 224 of circulator 220. Grating 230
receives the signals supplied by circulator 220 via second input/output
port 224. Grating 230 has a frequency-dependent transmission
characteristic similar to that described with reference to FIG. 2 wherein
optical input signals having wavelengths lying within a stop-band are
reflected by grating 230 and wavelengths falling outside the stop-band
pass through grating 230. The one or more optical signals supplied to
first port 232 of grating 230 which have optical wavelengths falling
within the second grating stop-band are reflected back toward second
input/output port 224 of circulator 220. These wavelength(s) correspond to
the signals to be dropped from the input optical signals received via
transmission path 205. The reflected signals propagate clockwise within
circulator 220 to drop port 226.
One or more optical channels to be added via add/drop multiplexer 200 are
supplied to add port 228 of circulator 220. The optical signal(s) to be
added correspond to optical channel(s) having wavelength(s) which fall
within the stop-band of grating 210, but outside the stop-band of second
grating 230. This prevents crosstalk interference from occurring between
the channel(s) to be added and channel(s) having corresponding wavelengths
included in the input signals. The optical add signal(s) propagate
clockwise within circulator 220 and exit via input/output port 222 to
fiber grating 210. Because the optical add signal(s) are on one or more
optical channel(s) having wavelength(s) which lie within the stop-band of
grating 210, the signals to be added are reflected back toward
input/output port 222 of circulator 220. The signals to be added propagate
clockwise within circulator 220 and exit via input/output port 224 onto
transmission path 206. Because the signal(s) to be added have
wavelength(s) which fall outside the stop-band of grating 230, the signals
to be added pass-through grating 230 via first port 232 and second port
234. Thus, the second fiber grating 230 passes the signal(s) to be added,
together with the optical input signal(s) having wavelengths which fall
outside the stop-band of second grating 230. In this manner, add/drop
multiplexer employs a single optical circulator to add/drop one or more
optical channels, thereby providing lower loss than add/drop multiplexers
which employ two optical circulators. Moreover, by preventing the one or
more optical signals to be added from entering circulator 220 utilizing
grating 210, crosstalk interference between channels to be added and
channels having corresponding wavelengths within the input signals is
avoided.
Operation of an optical communication system having an add/drop multiplexer
including a four-port optical circulator will be explained now with
reference to FIG. 3. The communication system 300 comprises an optical
input waveguide 310 which optically communicates with input port 322 of
first fiber Bragg grating 320. Optical waveguide 310 is configured to
receive input optical signals such as multiplexed optical signals having
channel spacings ranging from, for example, 25-200 GHz. Transmission path
310 is typically a single mode optical fiber, however any optical medium
capable of carrying multiplexed optical signals can be used. Grating 320
has a frequency dependent transmission characteristic wherein optical
channels having wavelengths which fall outside a stop-band pass through
the grating and optical channels having wavelengths which fall within the
stop-band are reflected by grating 320. An output port 324 of grating 320
optically communicates with first input/output port 332 of four-port
optical circulator 330. A drop port 336 of the four-port optical
circulator 330 optically communicates with waveguide 340. Second
input/output port 334 optically communicates with second fiber Bragg
grating 370.
Optical input waveguide 310 carries wavelength division multiplexed (WDM)
optical input signals 315, comprising N optical channels where each
optical channel i (i=1 to N) may carry optical signals operating on a
different optical wavelength .SIGMA..lambda..sub.1 . . . N. The WDM
optical input signals 315 are provided to input port 322 of the first
optical fiber grating 320 which reflects channels having wavelengths
within the stop-band of grating 320 and passes, to input/output port 332,
channels having wavelengths that fall outside the stop-band. Grating 320
is configured to reflect optical signals having wavelengths, for example
.lambda..sub.a, that correspond to channels to be added via waveguide 360
and add port 338. The channels falling outside the stop-band of grating
320 propagate in a clockwise direction within circulator 330 and exit via
second input/output port 334. Input 372 of second fiber Bragg grating 370
optically communicates with second input/output port 334 and receives the
optical signals transmitted through grating 320 as well as optical signals
365 added via port 338. The second grating 370 is configured to reflect
optical signals 345 on a selected optical channel having wavelength
.lambda..sub.d (where 1.ltoreq.d.ltoreq.N) which falls within a stop-band
associated with grating 370. The signals 345 are received by grating 370
and are reflected back toward second input/output port 334. The second
optical fiber grating 370 passes the optical signals falling outside the
grating stop-band to output port 374.
The four-port optical circulator 330 supplies the optical signals 345
having wavelength .lambda..sub.d reflected by fiber grating 370 to
waveguide 340 via drop port 336. Waveguide 340 supplies the signals 345 to
terminal 380 which may include, for example, photodetector 388 capable of
generating electrical signals in response to the received optical signals
345.
Waveguide 360 supplies the signals to be added 365 from terminal 385 which
may include, for example, a light source modulated to generate the optical
add signals at 365 having wavlength .lambda..sub.a. The add signals 365
are supplied to add port 338 of the optical circulator 330. The optical
add signals 365 have corresponding wavelengths that fall within the
stop-band of first grating 320, but outside the stop-band of second
grating 370. The add signals 365 propagate clockwise within circulator 330
to input/output port 332 and are reflected by first grating 320 back
toward input/output port 332. The add signals 365 propagates clockwise
within circulator 330 to second input/output port 334. Thus, waveguide 390
carries a WDM optical output signal having wavelengths
.SIGMA..lambda..sub.1 . . . N which comprises the add signals 365 and the
optical signals having wavelengths which lie outside of the stop-band
associated with grating 370. In this manner, a single four port optical
circulator is used to add optical channels having a first wavelength and
drop optical channels having a second wavelength within multiplexed
optical communication signals.
While the foregoing invention has been described in terms of the
embodiments discussed above, numerous variations are possible.
Accordingly, modifications and changes are considered to be within the
scope of the following claims.
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